daughter of Tartarus and Gaea.
She was half-woman, half-serpent, and mated with her brother, the monster
Typhon. They had the dogs of the underworld Cerberus
and Orthrus, the Hydra,
Chimera, the Sphinx
as well as the eagle that ate Prometheus
liver every day.
Echidna lived wih Typhon in Asia. She once stole the horses of Heracles, and would not return them until he slept with her. From this the triplets Agathyrsus, Gelanus and Scythes were born. The latter became the king of the Scythians.
Echidna was eventually killed in her sleep by Argus Panopes, a grandson of Argus.